Field of the Invention
The invention relates to a method for correcting a B0 field map measured with a magnetic resonance device, describing deviations from a nominal magnetic field strength in the homogeneity range of the magnetic resonance device by deviations from a nominal Larmor frequency for protons bonded into water, containing the deviations as Larmor frequency values for different picture elements in respect of chemical shifts of the Larmor frequencies, wherein the B0 field map has been recorded at least partly with spins of the protons bonded into fat and water that are not in phase, especially using a multi-echo method. The invention also relates to a magnetic resonance device and to a computer program.
Description of the Prior Art
Magnetic resonance imaging and its fundamentals are widely known. An object to be examined is introduced into a basic magnetic field with a relatively high field strength, known as the B0 field. In order to be able to acquire magnetic resonance data, in a slice of a subject for example, the nuclear spins of this slice are excited and the decay of this excitation is evaluated as a signal, for example. By operation of a gradient coil arrangement, gradient fields can be created, while radio-frequency excitation pulses, which are frequently referred to as radio-frequency pulses, are being radiated by a radio-frequency coil arrangement. Through the totality of the radio-frequency pulses (excitation), a radio-frequency field is created that is generally referred to as the B1 field, which flips (deflects) the spins of resonantly-excited nuclei locally resolved by the gradients by a so-called flip angle in relation to the magnetic field lines of the basic magnetic field. The excited spins of the nuclei then emit the radio-frequency signals, which can be picked up by suitable receive antenna, in particular also by the radio frequency coil arrangement itself, and can be further processed in order to be able to reconstruct magnetic resonance image data in this way.
In relation to the basic magnetic field however, even in the so-called homogeneity area of the magnetic resonance device in which the imaging is operated, deviations can occur that can lead to a degradation of the resulting image quality. The effective inhomogeneity of the B0 field is dependent on technical imperfections, but also on different susceptibility effects produced by the examination object, such as a patient, that differ individually. The reliable and robust measurement and imaging of this basic field recording continues to be the subject of current research.
To measure the basic magnetic field (B0 field), referred to as B0 mapping, magnetic resonance data are usually recorded (acquired) at two different echo times, preferably by gradient echo imaging. The phase difference (phase change) of the magnetic resonance data recorded at these different echo times, which can be established for example by subtraction of the phases of two magnetic resonance images of the first magnetic resonance data recorded at different echo times, is proportional to a deviation of the local B0 field from the nominal basic magnetic field strength and to the dephasing time, i.e. the difference of the two echo times. The field deviation is described in such cases by a deviation of the Larmor frequency from a nominal Larmor frequency for protons of the magnetic resonance device bonded to water (a variable describing this deviation is usually referred to below as the Larmor frequency value).
The phase created by deviations in the homogeneity of the B0 field thus develops over time, but the effect of the Nyquist phase wrapping must be taken into consideration, since the proportionality of the phase difference of magnetic resonance data recorded at different times to the deviation from the nominal Larmor frequency and the difference of the echo times applies only as long as a phase difference of 2π corresponds to the actual phase evolution. Depending on the dynamic range of the B0 distribution, the phases can continue to develop locally, however, by multiples of 2π. This leads to ambiguities and errors in the calculation of the B0 maps. Incorrect assignments in the phase evolution become evident in non-physical spatial jumps as a result of the 2π jumps in the phase difference images. This means, when the deviation of the local Larmor frequency from the nominal Larmor frequency is high, that an extremely fast development of the B0 phase also occurs, so that when the echo time (here the difference of the two echo times) is not short enough the phase will have extended beyond 2π, so that the described ambiguity occurs.
The selection of extremely short dephasing times is often not possible because of the sequences used wherein, with an extremely short echo time difference, smaller deviations from the nominal Larmor frequency can no longer be measured with sufficient accuracy.
A number of approaches are known for resolving the ambiguity problem in the assignment of the measured phase change. Thus it is possible to select the dephasing time, i.e. the difference between the echo times, so short that the phases do not develop by more than 2π at any location during this time. Since the dynamic range of the B0 field distribution is not known before the measurement however, the dephasing time must be selected so short that the sensitivity of the recording method is not sufficient and this method of operation is thus, as previously explained, not used.
It has therefore been proposed that phase jumps in the B0 maps be detected and corrected in post processing, assuming that the B0-field is spatially continuous. Algorithms that do this are referred to as phase unwrapping algorithms. However the reliability of such algorithms is frequently called into question. The main difficulty is because the overall volume can be composed of non-contiguous part regions, so that the individual part regions of the B0 maps are separated by voxels which only contain noise and are very signal-poor. Thus the phase in these voxels cannot be determined or can only be determined very unreliably.
It has also been proposed that magnetic resonance data be recorded with increasing dephasing time, thus with increasing difference between the echo times. The shortest dephasing time is selected in such cases so that no spatial phase jumps occur. From the recordings with shorter dephasing times it is estimated whether a phase jump will occur with a longer dephasing time. If so, this is taken into account in the evaluation (reconstruction) of the first magnetic resonance data with a longer dephasing time. The phase ambiguity is thus resolved and long dephasing times for a high sensitivity are made possible.
In an article by J. Dagher et al, “High-resolution, large dynamic range field map estimation”, Magn. Reson. Med. 71 (2014) 105-117, as well as other publications, so-called multi-echo methods have been described. The detection of echoes for different dephasing times in such methods has been shown to be the variant with the best prospects. In such cases methods have also been proposed in which, during a measurement process, i.e. after an excitation, a number of echoes were recorded so that different dephasing times are also produced for a single measurement. Of significance for a high-quality and reliable determination of the B0 field map is the selection of the dephasing times. In this context it is known from an article by Joseph Dagher et al., “A method for efficient and robust estimation of low noise, high dynamic range B0 maps”, Proc. Intl. Soc. Mag. Reson. Med. 20 (2012), Page 613, that an optimization approach can be used for determining the dephasing times which is based on simulated annealing, wherein other variants have also already been proposed.
With these multi-echo methods, i.e. methods with a number of different dephasing times, although the problem of phase wrap because of high off resonances is resolved, there is the additional problem of chemical shift when using variable echo time pairings. The chemical shift causes the resonant frequency of a spin ensemble to depend for a given B0 field on the chemical bonding. Therefore different Larmor frequencies with the same B0 field are produced for components of different chemical bonding, for example protons bonded into fat and water. Between fat and water this difference amounts to around 3.5 ppm.
Phase-based B0 mapping methods now measure how the spatial distribution of the deviations of the resonance frequencies from the nominal Larmor frequency of the magnetic resonance device were presented as a B0 map. The nominal Larmor frequency of a magnetic resonance device usually relates to protons bonded into water, so that the Larmor frequency values in the B0 field map only correspond to B0 deviations if the resonant frequency of water spins is considered. If both fat and also water areas exist in the examination volume for example, i.e. areas in which more protons bonded into fat or more protons bonded to water are present, a different Larmor frequency will be measured in these areas even with a constant B0 field. This problem occurs anew with the multi-echo methods since most other known B0 mapping methods typically carry out measurements “in phase”, meaning that the relevant phase positions of protons bonded into water and into fat coincide and do not lead to any phase difference, thus also do not lead to artifacts as a result of chemical shift. For example, B0 mapping methods are cited in the standard work by M. A. Bernstein, K. F. King and X. J. Zhou, “Handbook of MRI pulse sequences”, Elsevier Academic Press, 2004. In these methods, however, many phase wraps occur which are to be classified as critical.
If now, as with the multi-echo methods, different dephasing times are used, it can no longer be insured that the matching of the phase positions of protons bonded into fat and into water continues to exist, so that the chemical shift has an influence on the measurement of the B0 field map. Typically the chemical shift is expressed by a sudden noticeable incidence of the fat-containing regions within the B0 field map. In many applications, such as for example parallel transmission technology or in advanced reconstruction techniques in echoplanar imaging, only one frequency map or B0 field map of the water signal is needed. No correction method is currently available however which removes these sudden fat regions from the B0 field map. A simple segmentation through frequency thresholding would be conceivable, but not practicable, since the chemical shift of fat to water overlaps with the spectrum of susceptibility-related field inhomogeneities, so that a decision as to whether a high deviation from the nominal Larmor frequency is to be attributed to protons bonded into fat or to a field inhomogeneity cannot be made in a simple manner.